A wireless communication device includes various high-frequency elements such as an antenna, a filter, an RF switch, a power amplifier, a coupler, and a balun. Of these elements, the coupler, which is also called “Directional Coupler”, is used for picking up a part of an output of a power amplifier and feeding it back to an input of the power amplifier. The feedback using the coupler allows an output gain of the power amplifier to be maintained constant.
As the wireless communication device using the coupler, there can be exemplified a mobile communication device such as a mobile phone or a mobile terminal, a wireless LAN device, and the like. Because of remarkable progress in size reduction of these wireless communication devices, the coupler is required to achieve further reduction in size and thickness. An example of the coupler used in these devices is disclosed in Patent Documents 1 and 2.
Recently, in addition to the use in the power amplifier, the coupler comes to be used for the purpose of controlling an antenna tuner. In this case, the coupler plays a role of reducing sensing error or wiring loss in a transmission/reception section of the wireless communication device.
In a case where the coupler is used for the purpose of controlling the antenna tuner, sometimes the coupler is required to incorporate a resistor element serving as an attenuator so as to achieve connection to an element having a different impedance therefrom and to respond to an impedance disturbance. Patent Document 3 discloses an example of the resistor element of such a type (although it is an example of a terminating resistor element, instead of the coupler).